Environmental regulations restrict emissions of pollutants such as carbon monoxide, nitrogen oxides, hydrocarbons and particulate matter from combustion engines. Generally the exhaust gas is passed through a device called catalytic converter to reduce the pollutants [Ref. 3]. The catalytic converter is designed to have a large surface area coated with catalytic materials such as platinum which act on the pollutants. The catalytic converter performs several functions:
1. Reduction of carbon monoxide (CO): Carbon monoxide which is a poisonous gas, occurs due to partially burned carbon in a hydrocarbon fuel. Although there is plenty of oxygen to react with in air, CO in the exhaust gas does not readily oxidize into carbon dioxide (CO2). Catalytic materials help convert CO to CO2.2CO+PO2→2CO2  Eq. 1
2. Reduction of nitrogen oxides: Nitrogen gas which exist in the ambient atmosphere about 78% by volume is a relatively inert gas. However at high temperatures and pressures generated in a combustion chamber, some of it gets oxidized to form NO and NO2, which is generally referred to as NOx. NOx can create health and environmental problems. Catalytic materials help break down NOx gases back to nitrogen and oxygen.2NO→N2+O2  Eq. 22NO2→N2+2O2  Eq. 3
3. Reduction of hydrocarbons: Hydrocarbons are those molecules of the fuel which go through the combustion chamber either unburned or only partially burned. Typical examples are benzene (C6H6), butadiene (C4H6), acetaldehyde (CH3CHO), acrolein (H2C═CHCHO), and formaldehyde (HCHO) [Ref. 4]. Most are classified as carcinogens and they can create environmental problems such as ozone or smog, as well. Catalytic materials break down the hydrocarbons so that they can react with the oxygen in the atmosphere to form water and carbon dioxide. For example in the case of formaldehyde:HCHO+O2→CO2+H2O  Eq. 4
Reduction of particulate matter: PM refers to soot particles which consist of hydrocarbons, sulfates and metallic residues that originate from the fuel and engine lubricant. Catalytic converter is not very effective at treating such clusters. PM is typically reduced by means of a filter installed at the exhaust.